If you have a washer/drier hookup with out the washer/drier that trap may become dry and is usually the one forgotten when winterizing etc..
Also a sink/shower trap may be being siphoned dry due to a faulty vacuum breaker or pest nest clogged vent.
Last but not necessarily least if a vent pipe was used to route solar or sat tv wiring from the roof the seal where the wires exit the pipe may have failed.
Good luck finding the source.
George & Cathy
'08 Titanuim 28E33SA #458, Husky16K, XM Roady2, PD9260 conversion. '04 3500 Ram 5.9 DRW, Gauges, F'Tank Mods, Aux Tank/t'box, BrakeSmart, Cobra CB (19), XM Direct.